Many natural gas reservoirs contain relatively low percentages of hydrocarbons (less than 40%, for example) and high percentages of acid gases, principally carbon dioxide, but also hydrogen sulfide, carbonyl sulfide, carbon disulfide and various mercaptans. Removal of acid gases from well production in remote locations is desirable to provide conditioned or sweet, dry natural gas either for delivery to a pipeline, natural gas liquids recovery, helium recovery, conversion to liquid natural gas or nitrogen rejection.
Cryogenic distillation has been used to separate carbon dioxide from methane since the relative volatility between methane and carbon dioxide is reasonably high. The overhead vapor is enriched with methane and the bottoms product is enriched with carbon dioxide and other heavier hydrocarbons. Cryogenic distillation processing requires the proper combination of pressure and temperature to achieve the desired product recovery.
The distillation functions by countercurrent vapor-liquid contacting, with vapors rising to the top and liquids passing downward in a vertical column. Trays, plates, or packing are typically used to bring the two phases into equilibrium. The differences in the volatility of the constituents cause vapor-liquid exchange of constituents at the contacting surfaces. Heat is generally applied at the bottom of the column to generate the rising vapor. Some of the top vapor is typically condensed to provide reflux liquid which carries constituents of lower volatility downward.
Cryogenic distillation can encounter potential difficulties when the feed stream to the tower contains significant quantities of one or more constituents that can freeze (for example, more than about 2% carbon dioxide) at normal column operating conditions. When a gas containing large quantities of carbon dioxide encounters the process conditions of a cryogenic demethanizer, the carbon dioxide can potentially freeze, thereby plugging the trays or packing and preventing tower operation. A successful distillative process to separate methane from carbon dioxide and other hydrocarbons must deal with the potential formation of carbon dioxide solids.
In what has become known as the "Ryan/Holmes process", methane and carbon dioxide are separated in a distillation column. The Ryan/Holmes process involves operation of the distillation column at temperatures, compositions and pressures which produce a solids potential zone for carbon dioxide within the column. The term "solids potential zone" is used with the Ryan/Holmes process because, although conditions in the tower are such that carbon dioxide solids would normally occur, the Ryan/Holmes process prevents actual solids formation from occurring. This is achieved by introducing into the upper portion of the distillation column an additive to suppress formation of acid gas solids. The Ryan/Holmes additive, which is a non-polar material that is miscible with methane, may comprise ethane, propane, butane, pentane, and mixtures thereof. After the methane/carbon dioxide separation, the additive is recovered in another distillation column. A more detailed description of the Ryan/Holmes process is disclosed in U.S. Pat. Nos. 4,318,723; 4,383,842; 4,451,274; and 4,462,814.
In what has become known as the "CFZ process" (an acronym for "Controlled-Freeze-Zone" process), a process has been proposed to take advantage of the freezing potential of carbon dioxide in cryogenic distillation, rather than avoiding solid carbon dioxide. In the CFZ process, acid gas components are separated by cryogenic distillation through the controlled freezing and melting of carbon dioxide in a single column, without use of freeze-suppression additives. The CFZ process uses a cryogenic distillation column with a special internal section (CFZ section) to handle the solidification and melting of CO.sub.2. This CFZ section does not contain packing or trays like conventional distillation columns, instead it contains one or more spray nozzles and a melting tray. Solid carbon dioxide forms in the vapor space in the distillation column and falls into the liquid on the melting tray. Substantially all of the solids that form are confined to the CFZ section. The portions of the distillation column above and below the CFZ section of the column are similar to conventional cryogenic demethanizer columns. A more detailed description of the CFZ process is disclosed in U.S. Pat. Nos. 4,533,372; 4,923,493; 5,120,338; and 5,265,428.
In both the Ryan/Holmes process and the CFZ process, the cryogenic distillation column used for separation of methane and carbon dioxide typically requires refrigeration to keep the upper portion of the distillation column below about -56.7.degree. C. (-70.degree. F.), and potentially below about -95.6.degree. C. (-140.degree. F.). If the gas stream contains CO.sub.2 in concentrations such that the CO.sub.2 may freeze out as solids during the distillation operation, the refrigeration system that cools the fractionation column must either prevent freezing of CO.sub.2 solids or manage solids that are formed. The concentration of CO.sub.2 in a gas stream at which freezing can occur depends primarily on the gas components, temperature and pressure. Gas streams containing as little as 50 ppm CO.sub.2 under certain conditions can form CO.sub.2 solids. The potential for solids formation increases with increasing concentrations of CO.sub.2. For example, gas streams having more than about 6% CO.sub.2 have a high potential for CO.sub.2 freezing in cryogenic columns. In the past, because of this freezing potential, refrigeration for fractional distillation columns containing significant CO.sub.2 content was provided by liquid refrigerant in a closed-loop system, such as a cascaded propane-ethylene system, sometimes referred to as "external refrigeration."
While external refrigeration systems have high thermodynamic efficiencies and they can provide the cooling needed to reflux the distillation column, they require extra rotating equipment and means to store and make up the refrigerant. On offshore oil and gas production platforms that support distillation systems, external refrigeration systems have the additional disadvantage of increasing the space and weight requirements of the platform, substantially adding to the cost of the platform. In remote locations, importing refrigerant make up can be costly, and in some applications is impractical.
In the art of distillative fractionation of streams that contain potential solid formers, there is a substantially unfilled need for an improved process that minimizes, and potentially avoids, the need for external refrigerants and associated systems.